If you get around the window limitation (you can't actually open that many windows in mIRC afaik), you could probably do it at 1000/sec, but it would probably still be slow. The only way to know for certain would be to test it out. As mentioned, sockets should get you around that limitation.
I still think you should just look into one of the channel services that are available for your ircd. They will already have the functionality built in for many kinds of protection as well as other options and features that will likely be useful.
You also have to keep in mind that mIRC is still a client that has to connect to the network. It is possible for it to get disconnected and even for some person to force it to be disconnected through an attack. That's much easier to do with a client than with network services. If you don't want network services, then you really would be better off using an eggdrop bot as its connection will be more stable and it will be faster for what you need.
